The primary purpose of this research is to assess the consequences of drug dependence. Drug dependence is thought to have deleterious effects on the individual, producing severe social, behavioral, and medical consequences; however, efforts to quantify these effects have been hampered by a lack of information about the individual prior to drug use. As a result, it is often impossible to determine if an observed clinical condition is the result of drug dependence or is reflective of a preexisting condition. Discordant identical, or monozygotic, twins offer a unique means for assessing the adverse consequences of drug dependence. Because members of monozygotic twins are genetically identical, the study of twin pairs in which only one twin is drug dependent provides a powerful assessment of the effects of drug abuse while controlling for genetic variability. A secondary purpose of the study is the assessment of early experiential differences (both drug-related and non-drug related) to aid in the identification of non-shared environmental factors important in the development of drug dependence. Through a larger twin study of the genetic influences on drug dependence conducted collaboratively by ARC investigators and Johns Hopkins University, are identifying twin pairs discordant for drug dependence willing to participate in the ARC Discordant Twin Study. The drug-abusing member of each twin pair is housed at the ARC residential unit for a 3-week drug-free period prior to study in order to distinguish between acute effects and long-term consequences of drug use. After this period, a variety of factors associated with drug dependence are assessed including: medical disorders and various metabolic and cardiovascular effects, personality and psychiatric status, neuropsychological performance, and early environmental experiences. Within-pair comparisons in each of these domains will significantly increase our knowledge concerning etiological factors important in drug dependence as well as the effects of long-term drug abuse.